I've built a 2Gig raid0 array to boot from. I'm using all four disks
as raid elements (two live, two as spares). [...]

After that, I set the rest of the disks up in a raid5 config.

You could also consider having a separate 2GB boot partition on each
disk, without glueing them together into a RAID0. In normal use, you'd
keep them all identical, using rsync or dump|load or something, but you
would also have the ability to test upgrades by installing on only one
of the multiple boot partitions. (I do this with a two-disk system,
with a small boot partition on each disk, and a large RAID0 over "the
rest" of both disks.)